I recently acquired a Nikkor 260mm/F10 process camera lens via eBay to use with my 8x10 Toyo. Although it didn't come with a retaining ring (mind you, I paid just a little over $100 for it, so I'm not complaining!), I've successfully mounted it to a lensboard using the flange that adusts the aperture (it has four threaded holes) and so far, I'm very impressed by the quality of the images I've captured with it.
However, I have my doubts about whether this mounting method is really appropriate for a lens as heavy as this one. My question, then: Does anyone know what size of thread Nikon used for the retaining ring and, better still, a source where I might buy one so I don't have to have one machined? A call to Nikon's customer service department came up empty and as this was really a graphic arts lens, I'm not sure that blindly calling camera stores in search of one will prove to be productive.